Compiler error building compiler-rt for Fuchsia

[ilovepi]

Hi, we're seeing breakages building LLVM for Fuchsia, that appear to be caused by https://reviews.llvm.org/D116208 when building compiler-rt.

The error message is as follows:

FAILED: compiler-rt/lib/sanitizer_common/CMakeFiles/RTSanitizerCommon.x86_64.dir/sanitizer_platform_limits_posix.cpp.o 
/b/s/w/ir/x/w/staging/llvm_build/./bin/clang++ --target=x86_64-unknown-linux-gnu --sysroot=/b/s/w/ir/x/w/cipd/linux -DHAVE_RPC_XDR_H=1 -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/s/w/ir/x/w/llvm-project/compiler-rt/lib/sanitizer_common/.. --target=x86_64-unknown-linux-gnu -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wstring-conversion -Wmisleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -ffile-prefix-map=/b/s/w/ir/x/w/staging/llvm_build/runtimes/runtimes-x86_64-unknown-linux-gnu-bins=../staging/llvm_build/runtimes/runtimes-x86_64-unknown-linux-gnu-bins -ffile-prefix-map=/b/s/w/ir/x/w/llvm-project/= -no-canonical-prefixes -Wall -std=c++14 -Wno-unused-parameter -O2 -g -DNDEBUG  -fPIC -fno-builtin -fno-exceptions -fomit-frame-pointer -funwind-tables -fno-stack-protector -fno-sanitize=safe-stack -fvisibility=hidden -fno-lto -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta -O3 -gline-tables-only -Wno-gnu -Wno-variadic-macros -Wno-c99-extensions -nostdinc++ -Wno-format -fno-rtti -Wframe-larger-than=570 -Wglobal-constructors -UNDEBUG -MD -MT compiler-rt/lib/sanitizer_common/CMakeFiles/RTSanitizerCommon.x86_64.dir/sanitizer_platform_limits_posix.cpp.o -MF compiler-rt/lib/sanitizer_common/CMakeFiles/RTSanitizerCommon.x86_64.dir/sanitizer_platform_limits_posix.cpp.o.d -o compiler-rt/lib/sanitizer_common/CMakeFiles/RTSanitizerCommon.x86_64.dir/sanitizer_platform_limits_posix.cpp.o -c /b/s/w/ir/x/w/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp
/b/s/w/ir/x/w/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp:227:17: error: no member named '__glibc_reserved1' in '_libc_fpstate'
    if (fpregs->__glibc_reserved1[12] == FP_XSTATE_MAGIC1)
        ~~~~~~  ^
/b/s/w/ir/x/w/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp:229:22: error: no member named '__glibc_reserved1' in '_libc_fpstate'
             fpregs->__glibc_reserved1[13] - static_cast<const char *>(ctx);
             ~~~~~~  ^

[ilovepi]

It appears this code assumes glibc >= 2.26.

@frobtech suggested this as a potential fix:

--- a/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp      
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp      
@@ -224,9 +224,13 @@ namespace __sanitizer {                                    
 #    if SANITIZER_LINUX && SANITIZER_X64                                       
     // See kernel arch/x86/kernel/fpu/signal.c for details.                    
     const auto *fpregs = static_cast<ucontext_t *>(ctx)->uc_mcontext.fpregs;   
-    if (fpregs->__glibc_reserved1[12] == FP_XSTATE_MAGIC1)                     
+    // The member names differ across header versions, but the actual layout   
+    // is always the same.  So avoid using members, just use arithmetic.       
+    const uint32_t *after_xmm =                                                
+        reinterpret_cast<const uint32_t*>(fpregs + 1) - 24;                    
+    if (after_xmm[12] == FP_XSTATE_MAGIC1)                                     
       return reinterpret_cast<const char *>(fpregs) +                          
-             fpregs->__glibc_reserved1[13] - static_cast<const char *>(ctx);   
+             after_xmm[13] - static_cast<const char *>(ctx);                   
 #    endif                                                                     
     return sizeof(ucontext_t);                                                 
   }